Hard to answer since this would vary greatly.
1st) Is this a hosted solution? (Rackspace, etc.)
If so, one of their entry Linux servers would suffice. Just confirm storage especially dealing with the various media you are dealing with.
2nd) If not hosted, is this server going to be located in a data center or someone’s small office?
You will need to decide on either a rack-server (data center) or tower-based (office).
For number 2, you can purchase quite inexpensively a Dell Server with local storage to begin with and then as you grow, just add an external storage unit from PowerVaults all the way up to Equallogic platform.